Microorganisms

Microorganisms are very small living things that can only be seen using a microscope. They include bacteria, viruses, fungi and protozoa. Most microorganisms are harmless to humans, but some can cause disease.

Bacteria are single-celled microorganisms that come in many different shapes and sizes. Some types of bacteria can cause infections in humans, such as Streptococcus pneumoniae, which causes pneumonia, and Staphylococcus aureus, which causes skin infections. However, many types of bacteria are actually beneficial to humans. For example, Lactobacillus acidophilus is used to make yogurt and cheese, and Bifidobacterium bifidum is used in the manufacture of some types of bread.

Viruses are even smaller than bacteria and can only reproduce inside the cells of other organisms. Like bacteria, some viruses can cause disease in humans; examples include influenza virus, which causes the flu, and human immunodeficiency virus (HIV), which causes AIDS. However, not all viruses are harmful; for instance, bacteriophages (viruses that infect bacteria) are sometimes used to treat bacterial infections.

Fungi are microorganisms that includes yeasts (such as Saccharomyces cerevisiae) and molds (such as Aspergillus niger). Fungi can grow on food or in damp areas such as bathrooms; some fungi can cause diseases in humans such as athlete’s foot or ringworm. However, other fungi are actually beneficial to humans; yeast is used to make bread rise and mold is used to make certain cheeses such as Roquefort cheese.

Protozoa are single-celled animals that usually live in watery environments such as ponds or lakes. Some protozoans can cause disease in humans; for example Giardia lamblia can cause diarrhea while Entamoeba histolytica can cause amoebic dysentery . Nevertheless ,most protozoans do not pose a threat to human health .